Re: Locking in the clk API

[Saravana Kannan]

On 01/27/2011 12:43 PM, Russell King - ARM Linux wrote:
> On Thu, Jan 27, 2011 at 12:30:36PM -0800, Saravana Kannan wrote:
> > On 01/27/2011 12:54 AM, Russell King - ARM Linux wrote:
> > > On Wed, Jan 26, 2011 at 08:34:20PM -0800, Saravana Kannan wrote:
> > > > I'm not too familiar with serial/tty, does anyone know if the
> > > > .set_termios needs to be atmoic? If not, we could just change
> > > > cpm_uart/cpm_uart_core.c to use mutex instead of spinlock.
> > >
> > > The locking is there to protect against the interrupt handler accessing
> > > the port->* stuff (which seems to have been forgotten by the cpm driver).
> > >
> > > I don't see any reason why clk_set_rate() needs to be under the spinlock
> > > there - we need the reprogramming of the baud rate within the spinlock
> > > on 8250 because of DLAB bit hiding the data registers.  It's also a good
> > > idea that it _is_ within the spinlock along with uart_update_timeout()
> > > to ensure timeouts and the baud rate are updated together.
> >
> > For internal tree purposes, does .set_termios need to be atomic? Can it
> > grab mutexes instead of spinlock?
>
> I think I already answered that question above where I said "protect
> against the interrupt handler accessing the port->* stuff".
>
> > Going back to the topic, how about CPU freq drivers possibly using
> > clk_set_rate() to change freq? Do you think that's not the case or a
> > concern?
>
> CPUfreq drivers probably should busy-wait until the CPU PLL has locked
> if the CPU is allowed to continue running while the PLL relocks.  Some
> implementations will halt the CPU while the PLL is transitioning and
> that's really not unreasonable to do.
>
> I think some even require the code to be running out of SRAM and SDRAM
> remain untouched while the PLL is transitioning (omap maybe?)

Looks like you are confident to consider clk_set_rate() as sleepable. 
Can we add a comment to clk.h that says so?

Otherwise, there is no point in the suggesting the 
clk_prepare/unprepare() APIs.

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